1. Field of the Invention
The present invention relates to an FM (frequency modulation) tuner for receiving frequency-modulated signals.
2. Description of the Related Art
The quality of audio output reproduced by an FM tuner deteriorates in a multipath reception (multipath interference state), in which direct waves from a radio broadcast station and reflected waves from buildings and the like along the transmission path are received. An FM signal necessitates a broader frequency band for transmission than an AM signal, for example, in order to change the frequency of the carrier wave on the basis of audio signals, and the like. Accordingly, in a case in which a transmission signal having a target frequency is received by an FM tuner, the receiver is susceptible to interference (adjacent-channel interference) from other signals transmitted at frequencies close to the target frequency. This phenomenon can have adverse effects on the quality of detected audio signals. Multipath interference and adjacent-channel interference are also not desirable for RDS (Radio Data System), in which text data, and the like, are overlaid with the FM radio broadcast signal.
FIG. 3 is a block diagram describing the configuration of a conventional FM tuner. An RF (Radio Frequency) signal received by an antenna 2 is frequency converted to a first intermediate signal SIF1 having a first intermediate frequency (IF) fIF1, and the SIF1 is frequency converted to a second intermediate signal SIF2 having a second intermediate frequency fIF2. The SIF2 is input to an IFBPF 4. The IFBPF 4 is a bandpass filter with an fIF2 as a center frequency. The bandwidth WF is configured to be variable within a range of about 40 kHz to about 220 kHz, for example.
An FM signal that has passed through the IFBPF 4 is supplied to a limiter amp 6. The limiter amp 6 increases the amplitude of the FM signal to create a rectangular wave, and removes noise overlaid onto the FM signal. The FM signal that has been amplified and rendered into a rectangular wave is input to an FM detection circuit 8. The FM detection circuit 8 FM-detects an output signal of the limiter amp 6, and outputs a detection signal SDET.
A matrix circuit 10 extracts a sum signal (L+R) and a difference signal (L−R) of the left and right audio signals (L signal and R signal) from SDET, which is a stereo composite signal, and separates and outputs the L signal from the R signal in accordance with a matrix scheme.
An S-meter circuit 12 generates an SIF1-containing amplitude variation component signal SM-AC on the basis of the first intermediate signal SIF1, for example, and also smoothes the variation component using a low-pass filter (LPF) and generates a reception field strength signal SM-DC. Included in the variation component signal SM-AC is a component that corresponds to adjacent-channel interference and multipath interference.
A high-pass filter (HPF) 14, a detection circuit 16, and a comparison circuit 18 are provided as circuits for detecting the presence or absence of adjacent-channel interference and multipath interference on the basis of SM-AC. The HPF 14 is capable of switching the cutoff frequency fc according to whether one or the other of a frequency band component corresponding to adjacent-channel interference or a frequency band component corresponding to multipath interference is extracted. For example, when a component corresponding to adjacent-channel interference is extracted from SM-AC, fc is set to about 100 kHz. When a component corresponding to multipath interference is extracted from SM-AC, fc is set to about 50 kHz. The detection circuit 16 performs rectifying detection on a high-frequency component that has passed through the HPF 14, and the component is converted to a DC voltage VSQ. The comparison circuit 18 compares an output level VSQ of the detection circuit 16 with a reference voltage VREF set to a predetermined threshold value. For example, if VSQ>VREF, a predetermined voltage VH (H level) corresponding to a logical value of “1” is output as an SQ sensor signal SSQ indicating a determination result that either adjacent-channel interference or multipath interference has occurred. However, if VSQ≦VREF, a predetermined voltage VL (L level, VL<VH) corresponding to a logical value of “0” is output as SSQ indicating a determination result that neither adjacent-channel interference nor multipath interference has occurred. In an FM tuner supporting RDS, an AF search is performed that automatically selects a broadcast station with favorable reception conditions, and SSQ, for example, is utilized in assessment of reception conditions at the time of automatic channel selection.
Also, the VSQ corresponding to an adjacent-channel interference component output from the detection circuit 16 is input to a bandwidth control circuit 20 for controlling the bandwidth WF of the IFBPF 4. The bandwidth control circuit 20 narrows the WF in instances in which adjacent-channel interference has occurred, and reduces the effect of adjacent-channel interference on the output audio signal. Here, the VSQ is smoothed using a predetermined time constant in order to suppress the effect on the output audio signal from frequent narrowing and expansion of the WF. A time constant of about 2 mS (millisecond) can be provided, for example.
The degree of separation of a stereo signal generated by the matrix circuit 10 is controlled by a stereo separation level control circuit 22. The stereo separation level circuit 22 adjusts the relative strength of the difference signal (L−R) in relation to the sum signal (L+R) through a matrix process performed by the matrix circuit 10.
For example, when a weak field is present, the separation level is reduced in order to lower the noise offensive to the ear in stereo audio, and reproduction is performed as monaural audio. The separation level is also reduced when multipath interference is present in order to reduce the effects from multipath noise on the (L−R) signal. The control input to the stereo separation level control circuit 22 in order to perform the control is generated by a control input generation circuit 24 on the basis of the sensor signal SM-DC for reception field strength and the sensor signal SM-AC for multipath interference.
An HPF 26 and a detection circuit 28 are provided as circuits for extracting a component corresponding to multipath interference from SM-AC for the purpose of stereo separation level control. The HPF 26 extracts a component of the frequency band corresponding to multipath interference from the SM-AC and outputs the component to the detection circuit 28. The detection circuit 28 performs rectifying detection on the high-frequency component that has passed through the HPF 26, converts the result to a DC voltage VMP, and outputs the result to the control input generation circuit 24. The control input generation circuit 24 attenuates the SM-DC in accordance with the VMP. As a result, the weaker receiving electric field, the lower output voltage of the control input generation circuit 24; also, the smaller fluctuation component derived from the multipath interference, the lower output voltage. In addition, the stereo separation level control circuit 22 lowers the separation level in instances in which the output voltage of the control input generation circuit 24 is low, and raises the separation level in instances when the voltage is high. It is accordingly possible to reduce the sense of auditory discomfort from noise at times of weak field and from multipath interference.
During RDS receiving status, if reception conditions deteriorate, an AF search is performed, and a broadcast station with favorable reception conditions is automatically selected. It is preferred that the AF search be performed as quickly as possible in order to shorten the time interval during which reception is interrupted. The AF search needs to be completed within at least several millisecond.
However, if the SQ sensor signal SSQ discussed above is used for the AF search, respective detection results for adjacent-channel interference and multipath interference cannot be obtained simultaneously. Accordingly, a problem has arisen in terms of the difficulty with which a quicker AF search is performed. Shortening the AF search time has been particularly difficult in instances in which the above-mentioned time constant is applied to the VSQ for the purpose of WF control.